In the 1940s, Alfred Ezra Mirsky, working with colleague Arthur Pollister at the Rockefeller Institute for Medical Research (now the Rockefeller University) in New York, became the first person to develop a method of isolating the genetic material in animal cells called chromatin. Chromatin is made up of deoxyribonucleic acid (DNA), special proteins called histones, and other proteins. These materials combine to form chromosomes at the stage of development of a single cell just before it divides into two "sister" cells.

Mirsky's discovery led to his further discovery that each cell in the entire body has exactly the same amount of DNA--except for the sperm cell and egg cell, which have half the amount of DNA contained in other cells. When the egg and sperm cells unite, they become a single cell containing the full complement of DNA. Mirsky's discoveries led the way to understanding the nucleus of...